1. Field of the Invention
The present invention relates to an electric apparatus for a vehicle safety device, and more particularly to the actuation of recording apparatus for vehicle safety devices which protect the occupants in a vehicle during a crash or collision of the vehicle. This invention further relates to a recording apparatus for enabling the discrimination between a case where a so-called percussion cartridge which is an initiator of an air-bag device for vehicles (hereinafter simply referred to as a squib) has misfired due to a trouble caused by static electricity or mechanical shock and a case where a squib has fired properly after the actual collision of the vehicle.
2. Description of the Prior Art
The vehicle safety devices here relate to an air-bag device which is a bag-shaped device to be installed in front of the driver's seat or any other occupant's seat in a vehicle. When a squib fires upon occurrence of a crash of the vehicle, the air-bag device is inflated to give a cushioning effect, thus performing a function of absorbing the impact force caused by a collision of the vehicle driver or other occupants against the structural member in front of his seat, thereby protecting the occupants. The operating principle of a known air-bag device is as follows. In the actuation circuit of the air-bag device including crash sensors, which are collision detecting devices responsive to a deceleration caused by a crash of a vehicle, and a squib as triggering means, if the contacts of the crash sensors are closed upon collision of the vehicle, an external power supply unit or a back-up power supply (e.g., one or more capacitors or an equivalent auxiliary power supply) in place of the external power supply unit should the latter has become inoperative, supplies power to the squib to make it fire an explosive, thereby actuating a gas generator and utilizing the produced gas pressure directly or indirectly to inflate an air-bag. While as a matter of course a vehicle safety device should ensure absolute safety for the vehicle occupants and the device must ensure a proper operation at the instant of a collision of the vehicle, it is also necessary for the vehicle safety device to prevent the occurrence of its malfunction during a normal operation of the vehicle which would otherwise cause an unexpected accident endangering the driver and other occupants. Thus, the construction of the electric circuit used in the device must have very high reliability. For this purpose, a measure is taken such that the electric circuit has a reduced number of components and necessary redundancy or the electric circuit is provided with a malfunction preventing device.
An example of prior art techniques of the above type is the electric circuit used in an actuation recorder for vehicle occupant protecting apparatus disclosed in U.S. Pat. No. 3,890,594. In accordance with the apparatus of this prior art invention, the voltage across a squib is monitored by the above-mentioned electric circuit and the apparatus includes a switch SW.sub.2 which turns on by the actuation of the squib irrespective of whether the squib has fired properly or erroneously, a collision sensing switch SW.sub.1 which turns off in response to a collision of the vehicle (e.g., a device which is mounted at a front part of the vehicle and comprises an element made of glass or the like material and coated with an electrically conductive material so as to be broken due to an impact caused by a collision of the vehicle), and a fuse which acts as a recorder.
However, the prior art apparatus of the above construction have the following disadvantages:
(1) If the battery is disconnected due to an impact caused by a collision of the vehicle, usually an electric current sufficient for the firing action is supplied to the squib from the back-up power supply capacitor (approximately, several thousands .mu.F); however, after the firing by the squib, the voltage of the back-up capacitor becomes considerably low, so that it can no longer supply sufficient electric power to the electric circuit forming the switch SW.sub.2 and the fuse acting as a recorder.
(2) Though there is used the collision sensing switch SW.sub.1 which opens upon collision of the vehicle, if the magnitude of an impact force to which crash sensors are responsive differs from that which causes the collision sensing SW.sub.1 to be opened, it is possible that the collision sensing device will fail to perform properly.
(3) Since the collision sensing switch SW.sub.1 has to be used in addition to the crash sensors which are the components of the air-bag device for vehicles, the cost of the apparatus increases as a whole, but nevertheless its reliability deteriorates.
The present invention has been made with a view to overcoming the foregoing disadvantages of the prior art apparatus.